(1) Field of the Invention
The present invention relates to an optical writing device and an image forming device, and in particular to a technology for preventing deterioration of image quality caused by switching a light emission amount of a light-emitting element used in an optical writing device from one preset light amount to another.
(2) Description of the Related Art
Recently, as a line-type optical writing device that can be provided with low cost, a print head with a light-emitting unit including organic light-emitting diodes (hereinafter referred to as “OLEDs”) is proposed (such a print head is hereinafter referred to as an “OLED-PH”). OLEDs are known to have a characteristic such that the longer a total duration during which an OLED has emitted light (hereinafter simply referred to as a “light emission duration” of an OLED) becomes, the smaller the light amount emitted from the OLED when supplied with the same amount of driving current becomes. This decrease in light emission efficiency results in different pixels of an optical writing device emitting different light amounts. When this difference in light amount between pixels becomes as great as several percents, image unevenness in the form of stripes becomes no longer ignorable in printed images, and satisfactory image quality can no longer be achieved. Further, deterioration of an OLED over time progresses at a faster rate when the OLED is caused to emit a great light amount, and also, the rate of deterioration of an OLED over time changes depending upon the temperature around the OLED.
In view of this, for example, Japanese Patent Application Publication No. 2003-029710 discloses a technology for correcting a driving current amount supplied to each of a plurality of OLEDs by monitoring the light emission duration and the surrounding temperature of each OLED, and thereby ensuring that each OLED continues to emit a desired preset light amount. In addition, for example, Japanese Patent Application Publication No. 2005-329634 discloses a technology of monitoring a light emission duration of an OLED and boosting the driving voltage applied to the OLED when the light emission duration exceeds a predetermined duration. These conventional technologies enable suppressing the difference between light amounts of OLEDs occurring due to the deterioration of the OLEDS over time by correcting the amount of driving current supplied to the respective OLEDs. Thus, these conventional technologies prevent deterioration of image quality.
Typically, the amount of driving current supplied to an OLED is digitally controlled by using a digital to analog converter (DAC), and is increased step-by-step as the OLED deteriorate over time. For example, by increasing the amount of driving current supplied to an OLED by one least significant bit (LSB) each time the OLED has emitted light for a predetermined amount of time, it can be ensured that the amount of driving current actually supplied to the OLED (hereinafter referred to as an “actual current amount”) equals the amount of driving current required for causing the OLED to emit a desired light amount (hereinafter referred to as a “target current amount”). However, during a time period from when the actual current amount is increased as described above until when the actual current amount is to be increased next, the actual current amount stays the same while the target current amount gradually increases. Due to this, during this time period, there is a difference between the actual current amount and the target current amount.
Further, in a typical image forming device, for example, the speed at which recording sheets are conveyed may be switched between full speed and half speed, depending upon whether the recording sheets being used are those with regular thickness or those with greater thickness. When switching from one sheet conveyance speed to another, a rotation speed of the photoreceptor drum of the image forming device is changed. Further, it is also necessary to change the amount of light emitted by an OLED included in an optical writing device of the image forming device, or that is, the amount of driving current supplied to the OLED, in order to ensure that the amount of light emission of the OLED remains the same before and after the switching of sheet conveyance speed.
However, as described above, there may be a difference between an actual current amount and an expected current amount of an OLED. That is, at the point when switching the sheet conveyance speed from full speed to half speed, the actual current amount before the switching may differ from the target current amount before the switching. When such a difference is present, even when the actual current amount after the switching is calculated by accurately halving the actual current amount before the switching, the difference between the actual current amount and the target current amount may remain present after the switching.
Meanwhile, because driving current supplied to an OLED is digitally controlled, it is not always possible to accurately halve the actual current amount before the switching to calculate the actual current amount after the switching. Accordingly, even when the actual current amount and the target current amount are equal before the switching of conveyance speed, the actual current amount and the target current amount may differ after the switching.
Further, such difference between the actual current amount and the target current amount of an OLED may also be produced when the light emission amount of the OLED is changed for purposes other than the switching of sheet conveyance speed.